A fundamental challenge in addiction biology is how to connect the molecular mechanisms of opioid receptor regulation with the complex phenomenon of tolerance. As the target of clinically important pain analgesics such as morphine, the mu opioid receptor (MOR) has been the focus of numerous studies. While much pharmacological data is available on MOR, almost nothing is known about specific temporal and combinatorial patterns of receptor phosphorylation during the time course of cellular events. Intriguing preliminary data led to the hypotheses that phosphorylation patterns may be encoded on MOR in an agonist-specific manner, and that these phosphorylations may be a mechanism by which different cellular pathways are regulated. To address these hypotheses in intact cells, a newly developed mass spectrometric technique will be applied in concert with cell biology methods, focusing on the agonist-induced effects of morphine compared to that of the opiate peptide DAMGO. These studies on MOR regulatory mechanisms may ultimately be highly important towards developing new strategies for modulating the undesired effect of tolerance. [unreadable] [unreadable] [unreadable]